Sempervivum is a genus of about forty species of low-growing, succulent perennials in the stonecrop family (Crassulaceae), known to gardeners as houseleeks or "hens and chicks." Each plant takes the form of a tight, symmetrical rosette of thick, water-storing leaves, often pointed at the tip and arranged in dense, geometric whorls. Leaf color is remarkably variable across the genus and its many cultivars — bright green, grey-green, bronze, plum, or deeply purpled — and many species develop reddish margins when grown in strong sun. Several species are clothed in fine white hairs that, in S. arachnoideum and its kin, stretch a cobweb-like net across the rosette.
The rosettes are monocarpic: after several years of vegetative life a mature rosette pushes up a thick flowering stem in summer, bearing star-shaped, actinomorphic blooms with six or more narrow petals in shades of red, pink, yellow, purple, or white. The flowering rosette then dies. Long before that happens, however, each plant has produced a ring of small offset rosettes on short stolons around its base — the "chicks" — so a healthy clump continually replaces itself and spreads sideways into a dense, low mat.
Native from Morocco eastward through the Iberian and Alpine ranges and onward across the Carpathians, Balkans, Turkey, Armenia, and the Caucasus to Iran, Sempervivum is essentially a plant of sunny, stony mountains. Most species grow on rocky outcrops, screes, and crevices in subalpine and alpine zones, anchored by shallow roots in thin, gritty soil. That mountain ancestry is the source of their famous hardiness: the genus tolerates USDA zone 4 winters with ease and is well adapted to extremes of cold, heat, and drought, though it is less comfortable in hot, humid lowlands above zone 8 or 9.
Sempervivum spreads readily from its abundant offsets and, more slowly, from tiny seeds; the flowers are hermaphroditic but stagger their anther maturation in a way that limits self-pollination. The combination of compact form, evergreen color, near-indestructibility, and easy vegetative increase has made the genus a perennial favorite for rock gardens, dry stone walls, troughs, green roofs, and shallow containers across the temperate world.
Etymology
The genus name combines the Latin semper, "always," with vivus, "living" — a direct calque of the Ancient Greek ἀείζωον used by earlier herbalists for the same plants. The reference is to their persistence: rosettes endure drought, cold, and the death of individual plants by constantly producing offsets. The English common name "houseleek" pairs "house" with the Anglo-Saxon leac meaning "plant" — a reminder of the long tradition of growing these succulents on the roofs of houses.
Distribution
Sempervivum is native to a broad arc of mountain country running from Morocco in the west, through the Iberian Peninsula and the Alps, across the Carpathians and the Balkan ranges, into Turkey, Armenia, and the Caucasus, and on to northern Iran. Within this range individual species are often narrowly endemic — S. pittonii is restricted to a small area of eastern Austria, for instance — while widespread species such as S. tectorum, S. montanum, and S. arachnoideum span much of the Alpine system. Switzerland alone records eight native taxa, including the cobweb houseleek (S. arachnoideum), the common houseleek (S. tectorum), and the hybrid S. ×fauconnetii.
Ecology
Houseleeks are plants of sunny, exposed rock — outcrops, screes, crevices in stone walls, and thin gritty soils on subalpine and alpine slopes. Their succulent rosettes store water against drought, their tight habit reduces wind and sun exposure on the individual leaf surface, and shallow root systems exploit ephemeral moisture in cracks rather than competing in deep soil. The result is a genus comfortable in conditions hostile to most flowering plants: hard frost, baking summer sun, lean substrates, and rapid drying. Most species are hardy to at least USDA zone 4; tolerance falters at the warm end, where heat and humidity in zones 8–9 and above stress plants more reliably than cold.
Cultivation
Sempervivum is among the easiest succulents to grow in temperate gardens, provided the basics are right: full sun (with a little afternoon shade in hot climates), excellent drainage, and lean, gritty soil — well-drained compost cut with up to about half sand or grit is a typical recommendation. The genus is well adapted to drought and large temperature swings and resents rich soil, deep shade, and standing water. Houseleeks suit rock and gravel gardens, dry stone walls, sink and trough plantings, shallow containers, and green roofs, and they make a useful drought-tolerant ground cover on slopes and in difficult crevices. A handful of species with fleshier rosettes (for example S. erythraeum) appreciate a winter cover such as a cold frame or perspex sheet to keep excess rain off their crowns. Contact with the sap can occasionally cause skin irritation.
Propagation
Propagation is the easiest part of growing Sempervivum. Each mature rosette surrounds itself with a ring of smaller offsets — the "chicks" — borne on short stolons. These can be detached at almost any time during the growing season and simply pressed into a little gritty compost, where they root in days. Lifting and resetting offsets in spring is also the standard way to fill the gaps left by the death of a flowered rosette. Sempervivum can be raised from seed as well: the flowers are hermaphroditic, but their anthers mature in sequence to favor cross-pollination, so seed-grown plants from mixed plantings often show considerable variation in color and form.
Cultural Uses
Houseleeks have a long folk association with the home. In several European traditions, including Welsh practice, they were deliberately planted on thatch and tile roofs to ward off fire and lightning strikes and to bring health and prosperity to the household. This rooftop habit is the source of both the species epithet tectorum ("of roofs") and the English common name "houseleek" — literally a "house plant" in the older Anglo-Saxon sense of leac.
Taxonomy Notes
Sempervivum L. was established by Linnaeus in Species Plantarum (1753, vol. 1, p. 464) and remains the accepted genus name. It sits in the family Crassulaceae (subfamily Sempervivoideae), order Saxifragales. GBIF treats the genus as accepted with about forty species plus a substantial number of natural and garden hybrids (Sempervivum ×alatum, S. ×barbulatum, S. ×comollii, S. ×fauconnetii, and others), reflecting the genus's notorious propensity for crossing where ranges overlap.
